This invention will be described in two embodiments, a vehicular fan controller and a vehicular interior light dimmer switch.
Presently used vehicular fan controllers are mounted in the output air duct of the fan and incorporate several resistors of different value. The speed of the d.c. fan motor is controlled by placing one or more of the resistors in circuit with the fan motor. Because of the large numbers made each year and the potential for cost savings, a great deal of development has gone into vehicular fan controllers. Until recently, the standard fan motor controller used wound wire resistors which are relatively expensive and which tend to collect debris, such as lint or leaves, circulating in the air duct. Because the wound wire resistors get rather hot, there is a statistically significant fire hazard.
State of the art fan motor controllers incorporate thick film resistors printed on a non-conductive substrate and fall into two categories: those where the substrate is a porcelain coated steel substrate and those where the substrate is wholly ceramic. Typical fan motor controllers are found in U.S. Pat. Nos. 5,000,662; 5,192,940; 5,274,351; 5,291,174 and 5,408,575. Quite a few millions of thick film vehicular fan controllers are made annually.
Thick film resistor assemblies, although superior in many ways to wound wire resistance controllers, have their own set of difficulties. The motor controllers having porcelain coated steel substrates tend to be more expensive but dissipate heat better than the motor controllers having wholly ceramic substrates which are less expensive but which have more difficulty dissipating heat. The porcelain coatings are typically applied to the metal by firing a glass frit on the metal substrate at very high temperatures, e.g. approaching 850.degree. C.
Another application of this invention is in dimmer switches used in the interior of vehicles. For some time, a major automotive manufacturer has been making an interior dimmer switch comprising a plastic housing having a series of conductive elements ending in plug in terminals, a rotatable dimmer controller having several conductive arms continuously in contact with the conductive elements in the housing, an aluminum heat sink having a flat side and a finned side, and a planar ceramic substrate providing a series of fixed resistors. One side of the ceramic substrate abuts the heat sink and the other side provides the resistors which are in circuit with the dimmer controller arms. A thermally conductive paste provide good thermal contact between the ceramic substrate and the flat side of the heat sink. The heat generated by current passing through the resistors transfers to the heat sink and is dissipated through the finned side. Quite a few millions of dimmer controllers of this type are made each year.
Because of the large numbers made each year and the potential cost savings, a great deal of development and refinement has gone into dimmer controls of this type. Examples are found in U.S. Pat. Nos. 4,935,717; 5,119,063 and 5,181,313.
Also of interest relative to this invention are the disclosures in U.S. Pat. Nos. 2,179,566; 3,895,272; 4,119,937; 4,138,656; 4,404,237; 4,912,306; 5,035,836; 5,038,132; 5,119,063 and 5,181,313.